Catalyst for producing acrylic acids and acrylates

ABSTRACT

In one embodiment, the invention is to a catalyst composition, comprising vanadium and titanium. Preferably, the molar ratio of vanadium to titanium in an active phase of the catalyst composition is greater than 0.5:1.

FIELD OF THE INVENTION

The present invention relates generally to the production of acrylic acid. More specifically, the present invention relates to catalyst used in the production of acrylic acid via the condensation of acetic acid and formaldehyde.

BACKGROUND OF THE INVENTION

α,β-unsaturated acids, particularly acrylic acid and methacrylic acid, and the ester derivatives thereof are useful organic compounds in the chemical industry. These acids and esters are known to readily polymerize or co-polymerize to form homopolymers or copolymers. Often the polymerized acids are useful in applications such as superabsorbents, dispersants, flocculants, and thickeners. The polymerized ester derivatives are used in coatings (including latex paints), textiles, adhesives, plastics, fibers, and synthetic resins.

Because acrylic acid and its esters have long been valued commercially, many methods of production have been developed. One exemplary acrylic acid ester production process involves the reaction of acetylene with water and carbon monoxide. Another conventional process involves the reaction of ketene (often obtained by the pyrolysis of acetone or acetic acid) with formaldehyde. These processes have become obsolete for economic, environmental, or other reasons.

Another acrylic acid production method utilizes the condensation of formaldehyde and acetic acid and/or carboxylic acid esters. This reaction is often conducted over a catalyst. For example, condensation catalyst consisting of mixed oxides of vanadium and phosphorus were investigated and described in M. Ai, J. Catal., 107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987). The acetic acid conversions in these reactions, however, leave much room for improvement.

Thus, the need exists for improved processes for producing acrylic acid, and for improved catalyst capable of providing high acetic acid conversions in the formation of acrylic acid.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is to a catalyst composition, comprising vanadium and titanium. Preferably, the molar ratio of vanadium to titanium in an active phase of the catalyst composition is greater than 0.5:1. The inventive catalyst, in one embodiment, further comprises from 15 wt % to 45 wt % phosphorus; and from 30 wt % to 75 wt % oxygen.

In another embodiment, the present invention is to a process for producing acrylic acid. The process comprises the step of contacting acetic acid and a alkylenating agent over the above-identified catalyst under conditions effective to produce acrylic acid and/or acrylate. Preferably, the alkylenating agent is formaldehyde and the product is acrylic acid. In one embodiment the overall acetic acid conversion in the reaction is at least 3 mol % and the space time yield of acrylic acid and/or methyl acrylates is at least 3 grams per liter of catalyst per hour.

In another embodiment, the invention is to a process for producing the above-identified catalyst. The process comprises the steps of contacting an aqueous solution of TiP₂O₇ with a amount of a vanadium precursor to form a wet catalyst precursor. Preferably, the vanadium precursor is NH₄VO₃. The process further comprises the step of drying the wet catalyst precursor to faun a dried catalyst composition, which has a molar ratio of vanadium to titanium greater than 0.5:1.

DETAILED DESCRIPTION OF THE INVENTION Introduction

Production of unsaturated carboxylic acids such as acrylic acid and methacrylic acid and the ester derivatives thereof via most conventional processes have been limited by economic and environmental constraints. One of the more practical processes for producing these acids and esters involves the aldol condensation of formaldehyde and (i) acetic acid and/or (ii) ethyl acetate over a catalyst. Exemplary classes of conventional catalyst include binary vanadium-titanium phosphates, vanadium-silica-phosphates, and alkali metal-promoted silicas, e.g., cesium- or potassium-promoted silicas. The alkali metal-promoted silicas, however, have been known to exhibit only low to moderate activity when used in aldol condensation reactions. As a result, the alkali metal-promoted silicas typically require metal dopants, e.g., bismuth, lanthanum, lead, thallium, and tungsten, to improve catalyst performance.

Binary vanadium-titanium phosphates have been studied with regard to the condensation of acetic acid and formaldehyde (or a methanol/oxygen mixture) to form acrylic acid. Catalysts with a vanadium:titanium:phosphorus molar ratio of 1:2:x, where x is varied from 4.0 to 7.0, have traditionally shown that the catalyst activity decreases steadily as the phosphorus content increased (see, for example M. Ai, J. Catal., 107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987), discussed above). The highest selectivity with respect to the aldol condensation products, e.g., acrylic acid and methyl acrylate, was obtained where x was 6.0. With these catalysts, the molar ratio of vanadium to titanium was maintained at or below 1:2.

Vanadyl pyrophosphate ((VO)₂P₂O₇) catalysts have also been extensively studied. In particular, vanadyl pyrophosphates in combination with other phosphates, i.e., titanium pyrophosphate (TiP₂O₇), have been investigated and have shown significant catalytic activity in the condensation of carboxylic acids and esters with formaldehyde. Several studies have shown that vanadium-titanium-phosphorus ternary oxides may demonstrate good catalytic performance in the aldol condensation of acetic acid with methanol/formaldehyde. In comparison, TiO₂, V₂O₅—TiO₂, and TiO₂—P₂O₅ were not found to be as effective. The best performance of these combinations of (VO)₂P₂O₇ and TiP₂O₇, however, has been obtained with a (VO)₂P₂O₇:TiP₂O₇ molar ratio of 1:4. At the (VO)₂P₂O₇:TiP₂O₇ molar ratio of 1:4, the molar ratio of vanadium:titanium:phosphorus in the resulting composition is 1:2:5. Thus, the (VO)₂P₂O₇/TiP₂O₇ catalyst systems that provided the highest yields (under the respective reaction conditions) were those having a vanadium to titanium ratio of about 0.5:1.

Catalyst Composition

It has now been discovered that certain catalysts effectively catalyze the aldol condensation of a carboxylic acid with an alkylenating agent, e.g. a methylenating agent, such as formaldehyde to form an unsaturated acid. Preferably, the reaction is an aldol condensation reaction of acetic acid with formaldehyde to form acrylic acid. In one embodiment, the present invention is to a catalyst composition comprising vanadium and titanium. In one embodiment, the vanadium and titanium are present either in the elemental form or as a respective oxide or phosphate. The catalyst comprises an active phase, which comprises the components that promote the catalysis. As one example, the active phase comprises metals, phosphorus-containing compounds, and oxygen-containing compounds. Preferably, the molar ratio of vanadium to titanium in the active phase of the catalyst composition is greater than 0.5:1, e.g., greater than 0.75:1, greater than 1:1, or greater than 1.25:1. In terms of ranges, the molar ratio of vanadium to titanium in the inventive catalyst may range from 0.5:1 to 20:1, e.g., from 0.5:1 to 10:1, or from 1:1 to 10:1. Thus, unlike conventional catalysts that comprise vanadium and titanium at low molar ratios, the inventive catalyst have higher molar ratios of vanadium to titanium.

It has now been found that catalysts comprising vanadium and titanium in these amounts surprisingly achieve unexpectedly high acetic acid conversions. For example, depending on the temperature at which the acetic acid formation reaction is conducted, acetic acid conversions of at least 3 mol %, e.g., at least 20 mol %, e.g., at least 40 mol %, or at least 50 mol %, may be achieved with these catalyst compositions. This increase in acetic acid conversion is achieved while maintaining high selectivity to the desired acrylate such as acrylic acid or methyl acrylate. For example, selectivities to the desired acrylate (optionally acrylic acid or methyl acrylate) of at least 25 mol %, e.g., at least 50 mol % or at least 65 mol % may be achieved with the catalyst of the present invention.

The total amounts of vanadium and titanium in the catalyst compositions of the invention may vary widely so long as these metals are present in the above-described molar ratios. In some embodiments, for example, the catalyst comprises at least 1 wt % vanadium, e.g., at least 8 wt % or at least 13 wt %, based on the total weight of the catalyst including all components, e.g., the metals, supports, modifiers, binders, etc. The catalyst may comprise at least 5 wt. % titanium, e.g., at least 10 wt. % or at least 12 wt. %. In terms of ranges, the catalyst may comprise from 1 wt % to 40 wt % vanadium, e.g., from 8 wt % to 17 wt % or from 13 wt % to 16 wt % vanadium; and from 5 wt % to 40 wt % titanium, e.g., from 10 wt % to 16 wt % or from 10 wt % to 13 wt % titanium. The catalyst preferably comprises vanadium and titanium, in combination, in an amount greater than 6 wt %, e.g., greater than 18 wt % or greater than 23 wt %. In terms of ranges, the combined weight percentage of the vanadium and titanium components may range from 6 wt % to 80 wt %, e.g., from 18 wt % to 33 wt % or from 23 wt % to 29 wt %. The combined weight percentage of the vanadium and titanium may be greater than the combined weight percentage of vanadium and titanium in conventional catalyst.

In other embodiments, the inventive catalyst may further comprise other compounds or elements (metals and/or non-metals). For example, the catalyst may further comprise phosphorus and/or oxygen. In these cases, the catalyst may comprise from 15 wt % to 45 wt % phosphorus, e.g., from 20 wt % to 35 wt % or from 23 wt % to 27 wt %; and/or from 30 wt % to 75 wt % oxygen, e.g., from 35 wt % to 65 wt % or from 48 wt % to 51 wt %.

In some embodiments, the titanium is present in compound form such as in the form of titanium dioxide. For example, the catalyst may comprise titanium dioxide in an amount ranging from 0.1 to 95 wt %, e.g., from 5 wt % to 50 wt % or from 7 wt % to 25 wt %. In these cases, the titanium dioxide may be in the rutile and/or anatase form, with the anatase form being preferred. If present, the catalyst preferably comprises at least 5 wt. % anatase titanium dioxide, e.g., at least 10 wt. % anatase titanium dioxide, or at least 50 wt. % anatase titanium dioxide. Preferably less than 20 wt % of the titanium dioxide, if present in the catalyst, is in rutile form, e.g., less than 10 wt % or less than 5 wt %. In other embodiments, the catalyst comprises anatase titanium dioxide in an amount of at least 5 wt %, e.g., at least 10 wt % or at least 20 wt %. In another embodiment, the titanium is present in the form of amorphous titanium hydroxide gel, which is preferably converted to TiP₂O₇.

The titanium hydroxide gel may be prepared by any suitable means including, but not limited to, the hydrolysis of titanium alkoxides, substituted titanium alkoxides, or titanium halides. In other embodiments, colloidal titania sols and/or dispersions may be employed. In one embodiment, titania coated colloidal particles or supports are used as a source of titanium dioxide. The hydrous titania may be amorphous or may contain portions of anatase and/or rutile depending on preparation method and heat treatment.

Upon treatment with a phosphating agent, the various forms of titania may be converted to titanium phosphates and/or titanium pyrophosphates. In some cases, a portion of the titanium may be present as unconverted titania and, hence, will be present in the final catalyst as anatase or rutile forms.

Generally speaking, the proportion of the crystalline forms of titania present in the catalyst is dependent on the titanium precursor, the preparative method, and/or the post-phosphorylating treatment. In one embodiment, the amount of anatase and rutile present in the active phase of the catalyst is minimized. The amount of crystalline titania, however, may be high with only a thin shell of porous catalyst existing on the titania support.

In one embodiment, a pentavalent vanadium compound is reduced to form the catalyst. The reduced pentavalent compound may be combined with a phosphorus compound and, optionally, promoters under conditions effective to provide or maintain the vanadium in a valence state below +5 to form the active metal phosphate catalysts. Various reducing agents and solvents may be used to prepare these catalysts. Examples include organic acids, alcohols, polyols, aldehydes, and hydrochloric acid. Generally speaking, the choice of the metal precursors, reducing agents, solvents, sequence of addition, reaction conditions such as temperature and times, and calcination temperatures may impact the catalyst composition, surface area, porosity, structural strength, and overall catalyst performance.

In one embodiment, suitable vanadium compounds that serve as a source of vanadium in the catalyst contain pentavalent vanadium and include, but are not limited to, vanadium pentoxide or vanadium salts such as ammonium metavanadate, vanadium oxytrihalides, vanadium alkylcarboxylates and mixtures thereof.

In one embodiment, suitable phosphorus compounds that serve as a source of phosphorus in the catalyst contain pentavalent phosphorus and include, but are not limited to, phosphoric acid, phosphorus pentoxide, polyphosphoric acid, or phosphorus perhalides such as phosphorus pentachloride, and mixtures thereof.

In one embodiment, the active phase of the catalyst corresponds to the formula:

V_(a)Ti_(b)P_(c)O_(d)

wherein the letters a, b, c and d are the relative molar amounts (relative to 1.0) of vanadium, titanium, phosphorus and oxygen, respectively in the catalyst. In these embodiments, the ratio of a to b is greater than 0.5:1, e.g., greater than 0.75:1, greater than 1:1, or greater than 1.25:1. Preferred ranges for molar variables a, b, c, and d are shown in Table 1.

TABLE 1 Molar Ranges Molar Range Molar Range Molar Range a 2 to 8 2 to 6 2 to 5 b 4 to 8 4 to 7 4 to 6 c 20 to 30 20 to 28 23 to 26 d 30 to 70 30 to 60 37 to 58

In another embodiment, the active phase of the catalyst corresponds to the formula:

(VO)_(a)P_(b)O_(c)—Ti_(d)P_(e)O_(f)

wherein the letters a, b, c, d, e, and f are the relative molar amounts (relative to 1.0) of VO, phosphorus, and oxygen (in the (VO_(a)P_(b)O_(c) component); and the relative molar amounts of, titanium, phosphorus, and oxygen (in the Ti_(d)P_(e)O_(f) component), respectively in the catalyst. In these embodiments, the combination of b and e may range from 20 to 30, e.g., from 23 to 26, and/or the combination of a, c, and f may range from 30 to 65, e.g., from 37 to 58. Preferred ranges for molar variables a, b, c, d, e, and f are shown in Table 2.

TABLE 2 Molar Ranges Molar Range Molar Range Molar Range a 1 to 16 2 to 14  4 to 10 b 2 to 20 6 to 20  4 to 12 c 8 to 64 16 to 60  33 to 45 d 2 to 6  3 to 5  3 to 4 e 4 to 14 6 to 11 6 to 9 f 15 to 45  22 to 39  22 to 32

In some embodiments, the catalyst further comprises additional metals and/or metal oxides. These additional metals and/or metal oxides may function as promoters. If present, the additional metals and/or metal oxides may be selected from the group consisting of copper, molybdenum, tungsten, nickel, niobium, and combinations thereof. Other exemplary promoters that may be included in the catalyst of the invention include lithium, sodium, magnesium, aluminum, chromium, manganese, iron, cobalt, calcium, yttrium, ruthenium, silver, tin, barium, lanthanum, the rare earth metals, hafnium, tantalum, rhenium, thorium, bismuth, antimony, germanium, zirconium, uranium, cesium, zinc, and silicon and mixtures thereof. Other modifiers include boron, gallium, arsenic, sulfur, halides, Lewis acids such as BF₃, ZnBr₂, and SnCl₄. Exemplary processes for incorporating promoters into catalyst are described in U.S. Pat. No. 5,364,824, the entirety of which is incorporated herein by reference.

If the catalyst comprises additional metal(s) and/or metal oxides(s), the catalyst optionally may comprise additional metals and/or metal oxides in an amount from 0.001 wt % to 30 wt %, e.g., from 0.01 wt % to 5 wt % or from 0.1 wt % to 5 wt %. If present, the promoters may enable the catalyst to have a weight/weight space time yield of at least 25 grams of acrylic acid/gram catalyst-h, e.g., least 50 grams of acrylic acid/gram catalyst-h, or at least 100 grams of acrylic acid/gram catalyst-h.

In some embodiments, the catalyst is unsupported. In these cases, the catalyst may comprise a homogeneous mixture or a heterogeneous mixture as described above. In one embodiment, the homogeneous mixture is the product of an intimate mixture of vanadium and titanium resulting from preparative methods such as controlled hydrolysis of metal alkoxides or metal complexes. In other embodiments, the heterogeneous mixture is the product of a physical mixture of the vanadium and titanium phosphates. These mixtures may include formulations prepared from phosphorylating a physical mixture of preformed hydrous metal oxides. In other cases, the mixture(s) may include a mixture of preformed vanadium pyrophosphate and titanium pyrophosphate powders.

In another embodiment, the catalyst is a supported catalyst comprising a catalyst support in addition to the vanadium, titanium, and optionally phosphorous and oxygen, in the amounts indicated above (wherein the molar ranges indicated are without regard to the moles of catalyst support, including any vanadium, titanium, phosphorous or oxygen contained in the catalyst support). The total weight of the support (or modified support), based on the total weight of the catalyst, preferably is from 75 wt. % to 99.9 wt. %, e.g., from 78 wt. % to 97 wt. % or from 80 wt. % to 95 wt. %. The support may vary widely. In one embodiment, the support material is selected from the group consisting of silica, alumina, zirconia, titania, aluminosilicates, zeolitic materials, mixed metal oxides (including but not limited to binary oxides such as SiO₂—Al₂O₃, SiO₂—TiO₂, SiO₂—ZnO, SiO₂—MgO, SiO₂—ZrO₂, Al₂O₃—MgO, Al₂O₃—TiO₂, Al₂O₃—ZnO, TiO₂—MgO, TiO₂—ZrO₂, TiO₂—ZnO, TiO₂—SnO₂) and mixtures thereof, with silica being one preferred support. Other suitable support materials may include, for example, stable metal oxide-based supports or ceramic-based supports. Preferred supports include silicaceous supports, such as silica, silica/alumina, a Group IIA silicate such as calcium metasilicate, pyrogenic silica, high purity silica, silicon carbide, sheet silicates or clay minerals such as montmorillonite, beidellite, saponite, pillared clays, and mixtures thereof. Other supports may include, but are not limited to, iron oxide, magnesia, steatite, magnesium oxide, carbon, graphite, high surface area graphitized carbon, activated carbons, and mixtures thereof. These listings of supports are merely exemplary and are not meant to limit the scope of the present invention.

In other embodiments, in addition to the active phase and a support, the inventive catalyst may further comprise a support modifier. A modified support, in one embodiment, relates to a support that includes a support material and a support modifier, which, for example, may adjust the chemical or physical properties of the support material such as the acidity or basicity of the support material. In embodiments that use a modified support, the support modifier is present in an amount from 0.1 wt. % to 50 wt. %, e.g., from 0.2 wt. % to 25 wt. %, from 0.5 wt. % to 15 wt. %, or from 1 wt. % to 8 wt. %, based on the total weight of the catalyst composition.

In one embodiment, the support modifier is an acidic support modifier. In some embodiments, the catalyst support is modified with an acidic support modifier. The support modifier similarly may be an acidic modifier that has a low volatility or little volatility. The acidic modifiers may be selected from the group consisting of oxides of Group IVB metals, oxides of Group VB metals, oxides of Group VIB metals, iron oxides, aluminum oxides, and mixtures thereof. In one embodiment, the acidic modifier may be selected from the group consisting of WO₃, MoO₃, Fe₂O₃, Cr₂O₃, V₂O₅, MnO₂, CuO, Co₂O₃, Bi₂O₃, TiO₂, ZrO₂, Nb₂O₅, Ta₂O₅, Al₂O₃, B₂O₃, P₂O₅, and Sb₂O₃.

In another embodiment, the support modifier is a basic support modifier. The presence of chemical species such as alkali and alkaline earth metals, are normally considered basic and may conventionally be considered detrimental to catalyst performance. The presence of these species, however, surprisingly and unexpectedly, may be beneficial to the catalyst performance. In some embodiments, these species may act as catalyst promoters or a necessary part of the acidic catalyst structure such in layered or sheet silicates such as montmorillonite. Without being bound by theory, it is postulated that these cations create a strong dipole with species that create acidity.

Additional modifiers that may be included in the catalyst include, for example, boron, aluminum, magnesium, zirconium, and hafnium.

In some embodiments, the support may be a high surface area support, e.g., a support having a surface area of at least 1 m²/g, e.g., at least 20 m²/g or at least 50 m²/g, as determined by BET measurements. The catalyst support may include pores, optionally having an average pore diameter ranging from 5 nm to 200 nm, e.g., from 5 nm to 50 nm or from 10 nm to 25 nm. The catalyst optionally has an average pore volume of from 0.05 cm³/g to 3 cm³/g, e.g., from 0.05 cm³/g to 0.1 cm³/g or from 0.08 cm³/g to 0.1 cm³/g, as determined by BET measurements. Preferably, at least 50% of the pore volume or surface area, e.g., at least 70% or at least 80%, is provided by pores having the diameters discussed above. Pores may be formed and/or modified by pore modification agents, which are discussed below. In another embodiment, the ratio of microporosity to macroporosity ranges from 95:5 to 85:15, e.g., from 75:25 to 70:30. Microporosity refers to pores smaller than 2 nm in diameter, and movement in micropores may be described by activated diffusion. Mesoporosity refers to pores greater than 2 nm and less than 50 nm is diameter. Flow through mesopores may be described by Knudson diffusion. Macroporosity refers to pores greater than 50 nm in diameter and flow though macropores may be described by bulk diffusion. Thus, in some embodiments, it is desirable to balance the surface area, pore size distribution, catalyst or support particle size and shape, and rates of reaction with the rate of diffusion of the reactant and products in and out of the pores to optimize catalytic performance.

As will be appreciated by those of ordinary skill in the art, the support materials, if included in the catalyst of the present invention, preferably are selected such that the catalyst system is suitably active, selective and robust under the process conditions employed for the formation of the desired product, e.g., acrylic acid or alkyl acrylate. Also, the active metals that are included in the catalyst of the invention may be dispersed throughout the support, coated on the outer surface of the support (egg shell) or decorated on the surface of the support. In some embodiments, in the case of macro- and meso-porous materials, the active sites may be anchored or applied to the surfaces of the pores that are distributed throughout the particle and hence are surface sites available to the reactants but are distributed throughout the support particle.

The inventive catalyst may further comprise other additives, examples of which may include: molding assistants for enhancing moldability; reinforcements for enhancing the strength of the catalyst; pore-forming or pore modification agents for formation of appropriate pores in the catalyst, and binders. Examples of these other additives include stearic acid, graphite, starch, cellulose, silica, alumina, glass fibers, silicon carbide, and silicon nitride. Preferably, these additives do not have detrimental effects on the catalytic performances, e.g., conversion and/or activity. These various additives may be added in such an amount that the physical strength of the catalyst does not readily deteriorate to such an extent that it becomes impossible to use the catalyst practically as an industrial catalyst.

In one embodiment, the inventive catalyst composition comprises a pore modification agent. A preferred type of pore modification agent is thermally stable and has a substantial vapor pressure at a temperature below 300° C., e.g., below 250° C. In one embodiment, the pore modification agent has a vapor pressure of at least 0.1 KPa, e.g., at least 0.5 KPa, at a temperature between about 150° C. and about 250° C., e.g., between about 150° C. and about 200° C.

In some embodiments, the pore modification agent has a relatively high melting point, e.g., greater than 60° C., e.g., greater than 75° C., so that it does not melt during compression of the catalyst precursor into a slug, tablet, or pellet. Preferably, the pore modification agent comprises a relatively pure material rather than a mixture. As such, lower melting components will not liquefy under compression during formation of slugs or tablets. For example, where the pore modification agent is a fatty acid, lower melting components of the fatty acid mixtures may be removed as liquids by pressing. If this phenomenon occurs during slug or tablet compression, the flow of liquid may disturb the pore structure and produce an undesirable distribution of pore volume as a function of pore diameter on the catalyst composition. In other embodiments, the pore modification agents have a significant vapor pressure at temperatures below their melting points, so that they can be removed by sublimination into a carrier gas.

For example, the pore modification agent may be a fatty acid corresponding to the formula CH₃(CH₂)_(x)COOH where x>8. Exemplary fatty acids include stearic acid (x=16), palmitic acid (x=14), lauric acid (x=10), myristic acid (x=12). The esters of these acids and amides or other functionalized forms of such acids, for example, stearamide (CH₃(CH₂)₁₆CONH₂) may also be used. Suitable esters may include methyl esters as well as glycerides such as stearin (glycerol tristearate). Mixtures of fatty acids may be used, but substantially pure acids, particularly stearic acid, are generally preferred over mixtures.

In addition, while fatty acids and fatty acid derivatives are generally preferred, other compositions which meet the functional requirements discussed above are also suitable for use as pore modification agents. Other preferred pore modification agents include but are not limited to polynuclear organic compounds such as naphthalene, graphite, natural burnout components such as cellulose and its cellulosic derivatives, starches, natural and synthetic oligomers and polymers such as polyvinyl alcohols and polyacrylic acids and esters.

Catalyst Preparation

In some embodiments where the catalyst is unsupported, the catalyst composition is formed via a process comprising the step of contacting an aqueous solution of TiP₂O₇ with a predetermined amount of a vanadium precursor, e.g., a soluble NH₄VO₃, to form a wet catalyst precursor. Preferably, the process further comprises the step of drying the wet catalyst precursor to form a dried catalyst composition. The amounts of the TiP₂O₇ and the vanadium precursor are determined such that the resultant dried catalyst composition has a molar ratio of vanadium to titanium greater than 0.5:1, e.g., greater than 0.75:1, greater than 1:1, or greater than 1.25:1.

The process, in one embodiment, may further comprise calcining the dried catalyst, which, preferably, is conducted in accordance with a temperature profile. As one example, the temperature profile comprises an increasing stair step temperature profile comprising a plurality of increasing hold temperatures. The temperature increases at a rate from 1° C. to 5° C. per minute between said hold temperatures. Preferably, the hold temperatures comprise a first, second, third, and fourth hold temperature. The first hold temperature may range from 150° C. and 300° C., e.g., from 175° C. and 275° C., preferably being about 160° C. The second hold temperature may range from 250° C. and 500° C., e.g., from 300° C. and 400° C., preferably being about 250° C. The third hold temperature may range from 300° C. and 700° C., e.g., from 450° C. and 650° C., preferably being about 300° C. The fourth hold temperature may range from 400° C. and 700° C., e.g., from 450° C. and 650° C., preferably being about 450° C. Of course, other temperature profiles may be suitable. The calcination of the mixture may be done in an inert atmosphere, air or an oxygen-containing gas at the desired temperatures. Steam, a hydrocarbon or other gases or vapors may be added to the atmosphere during the calcination step or post-calcination to cause desired effects on physical and chemical surface properties as well as textural properties such as increase macroporosity.

In one preferred embodiment, the temperature profile comprises:

-   -   i) heating the dried catalyst from room temperature to 160° C.         at a rate of 10° C. per minute;     -   ii) heating the dried catalyst composition at 160° C. for 2         hours;     -   iii) heating the dried catalyst composition from 160° C. to         250° C. at a rate of 3° C. per minute;     -   iv) heating the dried catalyst composition at 250° C. for 2         hours;     -   v) heating the dried catalyst composition from 250° C. to         300° C. at a rate of 3° C. per minute;     -   vi) heating the dried catalyst composition at 300° C. for 6         hours;     -   vii) heating the dried catalyst composition from 300° C. to         450° C. at a rate of 3° C. per minute; and     -   viii) heating the dried catalyst composition at 450° C. for 2         hours.

In embodiments where the catalyst is supported, the catalyst compositions are formed through metal impregnation of a support (optionally modified support), although other processes such as chemical vapor deposition may also be employed.

In one embodiment, the catalysts are made by impregnating the support, with a solution of the metals or salts thereof in a suitable solvent, followed by drying and optional calcination. Solutions of the modifiers or additives may also be impregnated onto the support in a similar manner. The impregnation and drying procedure may be repeated more than once in order to achieve the desired loading of metals, modifiers, and/or other additives. In some cases, there may be competition between the modifier and the metal for active sites on the support. Accordingly, it may be desirable for the modifier to be incorporated before the metal. Multiple impregnation steps with aqueous solutions may to reduce the strength of the catalyst particles if the particles are fully dried between impregnation steps. Thus, it is preferable to allow some moisture to be retained in the catalyst between successive impregnations. In one embodiment, when using non-aqueous solutions, the modifier and/or additive are introduced first by one or more impregnations with a suitable non-aqueous solution, e.g., a solution of an alkoxide or acetate of the modifier metal in an alcohol, e.g., ethanol, followed by drying. The metal may then be incorporated by a similar procedure using a suitable solution of a metal compound.

In other embodiments, the modifier is incorporated into the composition by co-gelling or co-precipitating a compound of the modifier element with the silica, or by hydrolysis of a mixture of the modifier element halide with a silicon halide. Methods of preparing mixed oxides of silica and zirconia by sol gel processing are described by Bosman, et al., in J Catalysis, Vol. 148, (1994), page 660 and by Monros et al., in J Materials Science, Vol. 28, (1993), page 5832. Also, doping of silica spheres with boron during gelation from tetraethyl orthosilicate (TEOS) is described by Jubb and Bowen in J Material Science, Vol. 22, (1987), pages 1963-1970. Methods of preparing porous silicas are described in Her R K, The Chemistry of Silica, (Wiley, New York, 1979), and in Brinker C J & Scherer G W Sol-Gel Science published by Academic Press (1990).

The catalyst composition, in some embodiments, will be used in a fixed bed reactor for forming the desired product, e.g., acrylic acid or alkyl acrylate. Thus, the catalyst is preferably formed into shaped units, e.g., spheres, granules, pellets, powders, aggregates, or extrudates, typically having maximum and minimum dimensions in the range of 1 to 25 mm, e.g., from 2 to 15 mm. Where an impregnation technique is employed, the support may be shaped prior to impregnation. Alternatively, the composition may be shaped at any suitable stage in the production of the catalyst. The catalyst also may be effective in other forms, e.g. powders or small beads and may be used in these forms. In one embodiment, the catalyst is used in a fluidized bed reactor. In this case, the catalyst may be prepared via spray drying or spray thermal decomposition. Preferably, the resultant catalyst has a particle size of greater than 300 microns, e.g., greater than 500 microns.

Production of Acrylic Acid

In other embodiments, the invention is to a process for producing unsaturated acids, e.g., acrylic acids, or esters thereof (alkyl acrylates), by contacting an alkanoic acid with an alkylenating agent, e.g., a methylenating agent, under conditions effective to produce the unsaturated acid and/or acrylate. Preferably, acetic acid is reacted with formaldehyde in the presence of the inventive catalyst composition. The alkanoic acid, or ester of an alkanoic acid, may be of the formula R′—CH₂—COOR, where R and R′ are each, independently, hydrogen or a saturated or unsaturated alkyl or aryl group. As and example, R and R′ may be a lower alkyl group containing for example 1-4 carbon atoms. In one embodiment, an alkanoic acid anhydride may be used as the source of the alkanoic acid. In one embodiment, the reaction is conducted in the presence of an alcohol, preferably the alcohol that corresponds to the desired ester, e.g., methanol. In addition to reactions used in the production of acrylic acid, the inventive catalyst, in other embodiments, may be employed to catalyze other reactions. Examples of these other reactions include, but are not limited to butane oxidation to maleic anhydride, acrolein production from formaldehyde and acetaldehyde, and methacrylic acid production from formaldehyde and propionic acid.

The acetic acid may be derived from any suitable source including natural gas, petroleum, coal, biomass, and so forth. As examples, acetic acid may be produced via methanol carbonylation, acetaldehyde oxidation, ethylene oxidation, oxidative fermentation, and anaerobic fermentation. As petroleum and natural gas prices fluctuate, becoming either more or less expensive, methods for producing acetic acid and intermediates such as methanol and carbon monoxide from alternate carbon sources have drawn increasing interest. In particular, when petroleum is relatively expensive compared to natural gas, it may become advantageous to produce acetic acid from synthesis gas (“syn gas”) that is derived from any available carbon source. U.S. Pat. No. 6,232,352, which is hereby incorporated by reference, for example, teaches a method of retrofitting a methanol plant for the manufacture of acetic acid. By retrofitting a methanol plant, the large capital costs associated with carbon monoxide generation for a new acetic acid plant are significantly reduced or largely eliminated. All or part of the syn gas is diverted from the methanol synthesis loop and supplied to a separator unit to recover carbon monoxide and hydrogen, which are then used to produce acetic acid.

Methanol carbonylation processes suitable for production of acetic acid are described in U.S. Pat. Nos. 7,208,624, 7,115,772, 7,005,541, 6,657,078, 6,627,770, 6,143,930, 5,599,976, 5,144,068, 5,026,908, 5,001,259, and 4,994,608, all of which are hereby incorporated by reference.

U.S. Pat. No. RE 35,377, which is hereby incorporated by reference, provides a method for the production of methanol by conversion of carbonaceous materials such as oil, coal, natural gas and biomass materials. The process includes hydrogasification of solid and/or liquid carbonaceous materials to obtain a process gas which is steam pyrolyzed with additional natural gas to form syn gas. The syn gas is converted to methanol which may be carbonylated to acetic acid. U.S. Pat. No. 5,821,111, which discloses a process for converting waste biomass through gasification into syn gas, as well as U.S. Pat. No. 6,685,754 are hereby incorporated by reference.

In one optional embodiment, the acetic acid that is utilized in the condensation reaction comprises acetic acid and may also comprise other carboxylic acids, e.g., propionic acid, esters, and anhydrides, as well as acetaldehyde and acetone. In one embodiment, the acetic acid fed to the hydrogenation reaction comprises propionic acid. For example the propionic acid in the acetic acid feed stream may range from 0.001 wt. % to 15 wt. %, e.g., from 0.001 wt. % to 0.11 wt. %, from 0.125 wt. % to 12.5 wt. %, from 1.25 wt. % to 11.25, or from 3.75 wt. % to 8.75 wt. %. Thus, the acetic acid feed stream may be a cruder acetic acid feed stream, e.g., a less-refined acetic acid feed stream.

As used herein, “alkylenating agent” means an aldehyde or precursor to an aldehyde suitable for reacting with the alkanoic acid, e.g., acetic acid, in an aldol condensation reaction to form an unsaturated acid, e.g., acrylic acid, or an alkyl acrylate. In preferred embodiments, the alkylenating agent comprises a methylenating agent such as formaldehyde, which preferably is capable of adding a methylene group (═CH₂) to the organic acid. Other alkylenating agents may include, for example, acetaldehyde, propanal, and butanal.

The alkylenating agent, e.g., formaldehyde, may be added from any suitable source. Exemplary sources may include, for example, aqueous formaldehyde solutions, anhydrous formaldehyde derived from a formaldehyde drying procedure, trioxane, diether of methylene glycol, and paraformaldehyde. In a preferred embodiment, the formaldehyde is produced via a formox unit, which reacts methanol and oxygen to yield the formaldehyde.

In other embodiments, the alkylenating agent is a compound that is a source of formaldehyde. Where forms of formaldehyde that are not as freely or weakly complexed are used, the formaldehyde will form in situ in the condensation reactor or in a separate reactor prior to the condensation reactor. Thus for example, trioxane may be decomposed over an inert material or in an empty tube at temperatures over 350° C. or over an acid catalyst at over 100° C. to form the formaldehyde.

In one embodiment, the alkylenating agent corresponds to Formula I.

In this formula, R₅ and R₆ may be independently selected from C₁-C₁₂ hydrocarbons, preferably, C₁-C₁₂ alkyl, alkenyl or aryl, or hydrogen. Preferably, R₅ and R₆ are independently C₁-C₆ alkyl or hydrogen, with methyl and/or hydrogen being most preferred. X may be either oxygen or sulfur, preferably oxygen; and n is an integer from 1 to 10, preferably 1 to 3. In some embodiments, m is 1 or 2, preferably 1.

In one embodiment, the compound of formula I may be the product of an equilibrium reaction between formaldehyde and methanol in the presence of water. In such a case, the compound of formula I may be a suitable formaldehyde source. In one embodiment, the formaldehyde source includes any equilibrium composition. Examples of formaldehyde sources include but are not restricted to methylal (1,1 dimethoxymethane); polyoxymethylenes —(CH₂—O)_(i)— wherein i is from 1 to 100; formalin; and other equilibrium compositions such as a mixture of formaldehyde, methanol, and methyl propionate. In one embodiment, the source of formaldehyde is selected from the group consisting of 1, 1 dimethoxymethane; higher formals of formaldehyde and methanol; and CH₃—O—(CH₂—O)_(i)—CH₃ where i is 2.

The alkylenating agent may be used with or without an organic or inorganic solvent.

The term “formalin,” refers to a mixture of formaldehyde, methanol, and water. In one embodiment, formalin comprises from 25 wt % to 65% formaldehyde; from 0.01 wt % to 25 wt % methanol; and from 25 wt % to 70 wt % water. In cases where a mixture of formaldehyde, methanol, and methyl propionate is used, the mixture comprises less than 10 wt % water, e.g., less than 5 wt % or less than 1 wt %.

In some embodiments, the condensation reaction may achieve favorable conversion of acetic acid and favorable selectivity and productivity to acrylates. For purposes of the present invention, the term “conversion” refers to the amount of acetic acid in the feed that is converted to a compound other than acetic acid. Conversion is expressed as a mole percentage based on acetic acid in the feed. The conversion of acetic acid may be at least 30 mol %, e.g., at least 40%, or at least 50%. In another embodiment, the reaction may be conducted wherein the molar ratio of acetic acid to alkylenating agent is at least 0.55:1, e.g., at least 1:1.

Selectivity is expressed as a mole percent based on converted acetic acid. It should be understood that each compound converted from acetic acid has an independent selectivity and that selectivity is independent from conversion. For example, if 30 mole % of the converted acetic acid is converted to acrylic acid, the acrylic acid selectivity would be 30 mol %. Preferably, the catalyst selectivity to acrylates, e.g., acrylic acid and methyl acrylate, is at least 40 mol %, e.g., at least 50 mol %, at least 60 mol %, or at least 70 mol %. In some embodiments, the selectivity to acrylic acid is at least 30 mol %, e.g., at least 40 mol %, or at least 50 mol %; and/or the selectivity to methyl acrylate is at least 10 mol %, e.g., at least 15 mol %, or at least 20 mol %.

The term “productivity,” as used herein, refers to the grams of a specified product, e.g., acrylates, formed during the condensation based on the liters of catalyst used per hour. A productivity of at least 20 grams of acrylates per liter catalyst per hour, e.g., at least 40 grams of acrylates per liter catalyst per hour or at least 100 grams of acrylates per liter catalyst per hour, is preferred. In terms of ranges, the productivity preferably is from 20 to 500 grams of acrylates per liter catalyst per hour, e.g., from 20 to 200 per kilogram catalyst per hour or from 40 to 140 per kilogram catalyst per hour.

As noted above, the inventive catalyst provides for high conversions of acetic acid. Advantageously, these high conversions are achieved while maintaining selectivity to the desired acrylates, e.g., acrylic acid and/or methyl acrylate. As a result, acrylate productivity is improved, as compared to conventional productivity with conventional catalysts.

The acetic acid conversion, in some embodiments, may vary depending upon the reaction temperature. In one embodiment, for example, when the reaction temperature is approximately 340° C., the acetic acid conversion is at least 3%, e.g., at least 5%, at least 10%, at least 15% or at least 25%. The selectivity to acrylates is maintained at, for example, at least 50%, e.g., at least 60%, or at least 70%. Accordingly, the productivity, e.g., the space time yield, of acrylates is at least 3 grams per liter catalyst per hour, e.g., at least 5 grams per liter catalyst per hour or at least 10 grams per liter catalyst per hour, when the reaction temperature is approximately 340° C. In another embodiment, the productivity of acrylic acid is at least 2 grams per liter catalyst per hour, e.g., at least 3 grams per liter catalyst per hour or at least 5 grams per liter catalyst per hour, when the reaction temperature is approximately 340° C.

In another embodiment where the reaction temperature is approximately 355° C., the acetic acid conversion is at least 5%, e.g., at least 10%, at least 25% or at least 40%. The selectivity to acrylates is maintained at, for example, at least 50%, e.g., at least 60%, or at least 70%. Accordingly, the productivity of acrylates is at least 10 grams per liter catalyst per hour, e.g., at least 15 grams per liter catalyst per hour or at least 20 grams per liter catalyst per hour, when the reaction temperature is approximately 355° C. In another embodiment, the productivity of acrylic acid is at least 3 grams per liter catalyst per hour, e.g., at least 5 grams per liter catalyst per hour or at least 10 grams per liter catalyst per hour, when the reaction temperature is approximately 355° C.

In another embodiment where the reaction temperature is approximately 370° C., the acetic acid conversion is at least 15%, e.g., at least 30% or at least 40%. The selectivity to acrylates is maintained at, for example, at least 50%, e.g., at least 60%, or at least 70%. Accordingly, the productivity of acrylates is at least 30 grams per liter catalyst per hour, e.g., at least 40 grams per liter catalyst per hour or at least 50 grams per liter catalyst per hour, when the reaction temperature is approximately 355° C. In another embodiment, the productivity of acrylic acid is at least 20 grams per liter catalyst per hour, e.g., at least 30 grams per liter catalyst per hour or at least 40 grams per liter catalyst per hour, when the reaction temperature is approximately 370° C.

Preferred embodiments of the inventive process also have low selectivity to undesirable products, such as carbon monoxide and carbon dioxide. The selectivity to these undesirable products preferably is less than 30%, e.g., less than 20% or less than 10%. More preferably, these undesirable products are not detectable. Formation of alkanes, e.g., ethane, may be low, and ideally less than 2%, less than 1%, or less than 0.5% of the acetic acid passed over the catalyst is converted to alkanes, which have little value other than as fuel.

The alkanoic acid or ester thereof and alkylenating agent may be fed independently or after prior mixing to a reactor containing the catalyst. The reactor may be any suitable reactor. Preferably, the reactor is a fixed bed reactor, but other reactors such as a continuous stirred tank reactor or a fluidized bed reactor, may be used.

In some embodiments, the alkanoic acid, e.g., acetic acid, and the alkylenating agent, e.g., formaldehyde, are fed to the reactor at a molar ratio of at least 0.50:1, e.g., at least 0.75:1 or at least 1:1. In terms of ranges the molar ratio of alkanoic acid to alkylenating agent may range from 0.50:1 to 10:1 or from 0.75:1 to 5:1. In some embodiments, the reaction of the alkanoic acid and the alkylenating agent is conducted with a stoichiometric excess of alkanoic acid. In these instances, acrylate selectivity may be improved. As an example the acrylate selectivity may be at least 10% higher than a selectivity achieved when the reaction is conducted with an excess of alkylenating agent, e.g., at least 20% higher or at least 30% higher. In other embodiments, the reaction of the alkanoic acid and the alkylenating agent is conducted with a stoichiometric excess of alkylenating agent.

The condensation reaction may be conducted at a temperature of at least 250° C., e.g., at least 300° C., or at least 350° C. In team of ranges, the reaction temperature may range from 200° C. to 500° C., e.g., from 250° C. to 400° C., or from 250° C. to 350° C. Residence time in the reactor may range from 1 second to 200 seconds, e.g., from 1 second to 100 seconds. Reaction pressure is not particularly limited, and the reaction is typically performed near atmospheric pressure. In one embodiment, the reaction may be conducted at a pressure ranging from 0 KPa to 4100 KPa, e.g., from 3 KPa to 345 KPa, or from 6 to 103 KPa.

Water may be present in amounts up to 60 wt %, by weight of the reaction mixture, e.g., up to 50 wt % or up to 40 wt %. Water, however, is preferably reduced due to its negative effect on process rates and separation costs.

In one embodiment, an inert or reactive gas is supplied to the reactant stream. Examples of inert gases include, but are not limited to, nitrogen, helium, argon, and methane. Examples of reactive gases or vapors include, but are not limited to, oxygen, carbon oxides, sulfur oxides, and alkyl halides. When reactive gases such as oxygen are added to the reactor, these gases, in some embodiments, may be added in stages throughout the catalyst bed at desired levels as well as feeding with the other feed components at the beginning of the reactors.

In one embodiment, the unreacted components such as the carboxylic acid and formaldehyde as well as the inert or reactive gases that remain are recycled to the reactor after sufficient separation from the desired product.

When the desired product is an unsaturated ester made by reacting an ester of an alkanoic acid ester with formaldehyde, the alcohol corresponding to the ester may also be fed to the reactor either with or separately to the other components. For example, when methyl acrylate is desired, methanol may be fed to the reactor. The alcohol, amongst other effects, reduces the quantity of acids leaving the reactor. It is not necessary that the alcohol is added at the beginning of the reactor and it may for instance be added in the middle or near the back, in order to effect the conversion of acids such as propionic acid, methacrylic acid to their respective esters without depressing catalyst activity.

EXAMPLES Example 1

Various catalyst compositions were prepared using TiP₂O₇ and NH₄VO₃ as reactants. An aqueous suspension of TiP₂O₇ was prepared by adding the finely powdered solid TiP₂O₇ to 50 mL of deionized water. A calculated amount of phosphoric acid (85%) was added and the suspension was heated to 80° C. with stirring and kept at this temperature for 30 minutes. Separately, oxalic acid was dissolved in deionized water, and this solution was heated to about 50° C. with stirring. The calculated amount of NH₄VO₃ was added to the oxalic acid solution in small portion over approximately 10 minutes. This solution was heated to 80° C. with stirring and kept at this temperature for 60 minutes. The oxalic acid solution was then added to the suspension of TiP₂O₇ and stirred. This final mixture was stirred for another 30 minutes at this temperature. The final mixture was then evaporated to dryness in a rotary evaporator. The resulting solid was air dried overnight at 120° C. and calcined using the following temperature profile:

-   -   i) heating from room temperature to 160° C. at a rate of 10° C.         per minute;     -   ii) heating at 160° C. for 2 hours;     -   iii) heating from 160° C. to 250° C. at a rate of 3° C. per         minute;     -   iv) heating at 250° C. for 2 hours;     -   v) heating from 250° C. to 300° C. at a rate of 3° C. per         minute;     -   vi) heating at 300° C. for 6 hours;     -   vii) heating from 300° C. to 450° C. at a rate of 3° C. per         minute; and     -   viii) heating at 450° C. for 2 hours.

Respective stoichiometric amounts of the reactants were calculated and utilized to achieve various catalyst compositions having the vanadium:titanium molar ratios shown in Table 3.

TABLE 3 Catalyst Compositions Vanadium:Titanium Catalyst Sample Catalyst Formula Molar Ratio A TiP₂O₇ — B TiP_(2.12)O_(7.26) — C V₂Ti₈P₁₉O_(67.5) 0.25:1 D V₂Ti₄P_(10.5)O₃₉  0.5:1 E V₃Ti₄P1_(1.5)O_(38.75) 0.75:1 F V₂Ti₂P_(6.24)O_(23.6)   1:1 G V₅Ti₄P_(13.5)O_(23.75) 1.25:1

A reaction feed comprising acetic acid, formaldehyde, methanol, water, oxygen, and nitrogen was passed through a fixed bed reactor comprising the catalyst shown in Table 3. The reactor temperature was 370° C. Acrylic acid and methyl acrylate (collectively, “acrylates”) were produced. The conversions, selectivities, and space time yields are shown in Table 4.

TABLE 4 Acrylate Production Acetic Acid Acrylate Acrylate Space Time Yield, Catalyst Sample Conversion Selectivity g/liter of catalyst/hr A 26.1 69.2 73.3 B 26.1 76.7 76.0 C 16.0 61.9 39.6 D 5.9 61.6 17.6 E 16.3 62.7 48.9 F 36.2 63.6 97.1 G 52.5 63.3 131.7

As shown in Table 4, acrylic acid conversion is at a minimum when the vanadium:titanium molar ratio is 0.5. Surprisingly and unexpectedly, as the amount of vanadium in the catalyst composition increases relative to the amount of titanium, e.g., as the vanadium:titanium molar ratio increases, the acetic acid conversion increases significantly. This result is in direct contrast to the teachings of the references relating to conventional condensation catalyst, which teach that the optimal vanadium:titanium molar ratio is 0.5:1. As shown in Table 4, the selectivity to acrylates is maintained even as the vanadium:titanium molar ratio is increased. The combination of higher acetic acid conversion and maintained acrylate selectivity surprisingly and unexpectedly provides for a significant increase in acrylate space time yield. For a vanadium:titanium molar ratio of 0.75, a 177% increase in space time yield was demonstrated, as compared to the space time yield for a conventional catalyst having a vanadium:titanium molar ratio of 0.5. For a vanadium:titanium molar ratio of 1.0 and 1.25, a 450% increase and a 650% increase over the space time yields for conventional catalysts, respectively, were demonstrated. Such significant improvements could not have been predicted based on the teachings of the references relating to conventional condensation catalyst.

While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description, the disclosures of which are all incorporated herein by reference. In addition, it should be understood that aspects of the invention and portions of various embodiments and various features recited below and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by one of skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention. 

1. A catalyst composition, comprising: vanadium and titanium, wherein the molar ratio of vanadium to titanium in an active phase of the catalyst composition is greater than 0.5:1.
 2. The composition of claim 1, wherein the catalyst comprises from 1 wt % to 40 wt % vanadium.
 3. The composition of claim 1, wherein the catalyst comprises from 5 wt % to 40 wt % titanium.
 4. The composition of claim 1, wherein the catalyst further comprises: from 15 wt % to 45 wt % phosphorus; and from 30 wt % to 75 wt % oxygen.
 5. The composition of claim 1, wherein the catalyst comprises at least 1 wt % vanadium.
 6. The composition of claim 1, wherein the catalyst comprises at least 5 wt % titanium.
 7. The composition of claim 1, wherein the catalyst comprises at least 6 wt % vanadium and titanium, combined.
 8. The composition of claim 1, wherein the catalyst further comprises a support.
 9. The composition of claim 8, wherein the support is selected from the group consisting of silica, alumina, zirconia, titania, aluminosilicates, zeolitic materials, and mixtures thereof.
 10. The composition of claim 1, wherein the catalyst is homogeneous.
 11. The composition of claim 1, wherein the catalyst is heterogeneous.
 12. The composition of claim 1, wherein the catalyst further comprises a binder.
 13. The composition of claim 1, wherein the catalyst corresponds to the formula V_(a)Ti_(b)P_(c)O_(d) wherein the ratio of a to b is greater than 0.5:1.
 14. The composition of claim 1, wherein: a is from 2 to 8; b is from 4 to 8; c is from 20 to 30; and d is from 30 to
 70. 15. A catalyst composition corresponding to the formula (VO)_(a)P_(b)O_(c)—Ti_(d)P_(e)O_(f) wherein a is from 1 to 16; b is from 6 to 20; c is from 8 to 64; d is from 2 to 6; e is from 4 to 14; f is from 15 to 45; (b+e) is from 20 to 30; and (a+c+f) is from 30 to
 65. 16. A process for producing acrylic acid, the process comprising the steps of: contacting an alkanoic acid and a alkylenating agent over a catalyst under conditions effective to produce acrylic acid and/or acrylate, wherein the catalyst comprises vanadium and titanium, wherein the molar ratio of vanadium to titanium is greater than 0.5:1.
 17. The process of claim 16, wherein the alklylenating agent comprises a methylenating agent.
 18. The process of claim 17, wherein the methylenating agent comprises formaldehyde.
 19. The process of claim 16, wherein the alkanoic acid comprises a acetic acid.
 20. The process of claim 16, wherein the molar ratio of alkanoic acid to alkylenating agent is at least 0.50:1.
 21. The process of claim 16, wherein contacting comprises reacting the alkanoic acid and the alkylenating agent at a stoichiometric excess of acetic acid.
 22. The process of claim 16, wherein the overall alkanoic acid conversion is at least 3 mol %.
 23. The process of claim 16, wherein the space time yield of the combination of acrylic acid and acrylates is at least 3 grams per liter of catalyst per hour.
 24. A process for producing a catalyst composition, the process comprising the steps of: contacting an aqueous solution of TiP₂O₇ with a amount of a vanadium precursor to form a wet catalyst precursor, drying the wet catalyst precursor to form a dried catalyst composition having a molar ratio of vanadium to titanium greater than 0.5:1.
 25. The process of claim 24, wherein the vanadium precursor is NH₄VO₃.
 26. The process of claim 24, further comprising calcining the dried catalyst in accordance with a temperature profile to form a dried catalyst composition.
 27. The process of claim 26, wherein the temperature profile comprises an increasing stair step temperature profile comprising a plurality of increasing hold temperatures, and wherein the temperature increases at a rate from 1 to 5° C. per minute between said hold temperatures.
 28. The process of claim 27, wherein the hold temperatures comprise a first hold temperature between 150° C. and 300° C., a second hold temperature between 250° C. and 500° C.
 29. The process of claim 28, wherein the hold temperatures further comprise a third hold temperature between 300° C. and 700° C.
 30. The process of claim 26, wherein the temperature profile comprises a constant temperature ramp wherein the temperature increases at a rate from 1 to 5° C. per minute. 